This invention relates to hermetically sealed heat engine powered devices, and more particularly to a free-piston Stirling engine driven alternator/compressor contained entirely within a single hermetically sealed casing. This invention is related to U.S. Patent Application Ser. Nos. 168,714 and 168,075 filed July 14, 1980, and the disclosures thereof are incorporated by reference herein.
The high theoretical efficiency of the Stirling engine has attracted considerable interest in the era of increasing fuel cost and decreasing fuel supplies. The omnivorous external combustor of the closed cycle Stirling engine adds the additional advantages of easy control of combustion emissions, use of safer, cheaper, and more readily available fuels, and quiet running operation, all of which combine to make the Stirling engine a highly desirable alternative to the internal combustion engine.
Despite these known advantages, development of the Stirling engine has proceeded at a much slower rate than would be expected. Certain of the problems that have been encountered are of such extreme difficulty as to cause resourceful and sophisticated organizations to abandon altogether the development of the Stirling engine. Some of the most intractable problems are the need to seal the working gas at high pressure within the working space, the requirement for transferring heat at high temperature from the heat source to the working gas through the heater head, and a simple, reliable and inexpensive means for modulating the power as the load changes.
One fruitful approach to the solution of these problems which is suitable for a certain range of applications is the free-piston Stirling engine. The free-piston Stirling engine uses a displacer which is mechanically independent of the power output member. Its motion and phasing relative to the power output member is accomplished by the state of a balanced dynamic system of springs and masses, rather than a mechanical linkage.
One technique for phasing the displacer and providing motive power to maintain the oscillating movement of the displacer to supply the energy dissipated by the displacer in shifting working fluid during the Stirling cycle is the use of a gas spring between the displacer and the power output member. This is a convenient means for maintaining the oscillating movement of the displacer but it produces an undesirable power coupling between the power output member and the displacer and it necessitates the use of undesirable close manufacturing tolerances in the area of the gas spring. In addition, the inherent hysteresis losses in a gas spring, representing an undesirable power loss, are magnified in such an arrangement.
One major advantage of the free-piston Stirling engine is its adaptability to hermetic sealing. This eliminates or simplifies many of the sealing problems and simplifies the mechanical design, resulting in a potential savings in fabrication cost and reduction of friction losses. It also offers the potential advantage of long-term, maintenance-free operation. However, in order to achieve this potential, the components having inherent short-term durability, such as those incorporating seals and sliding wear surfaces, must be strengthened or eliminated by redesign.